Last Duke Standing by Cheryl Bolen

As the third son of the Duke of Fordham, Alex never thought he’d become a duke. But he’s suddenly catapulted to the lofty title after his slightly older brother dies in his sleep. Now Alex has the onerous task of announcing the death to the woman his brother was to wed.
Ever pragmatic, Lady Georgiana Fenton insists on seeing the late duke’s body, and when she does, she’s convinced he was smothered as he slept. She and the new duke decide to secretly work together to uncover the murderer. But the longer they’re together, the harder it becomes to resist the duke’s scorching kisses–and even harder to dismiss him from the list of suspects. No one had more to gain by her fiancé’s death. . .